1. Field of Invention
This invention relates to battery chargers and more particularly to generation of a duty cycle signal for use in controlling switches in a battery charger to control current flow to a battery being charged by the battery charger while maintaining a high power factor at an AC input of the battery charger.
2. Description of Related Art
In conventional battery chargers, AC line voltage is stepped down by a transformer to produce a low voltage AC source which is connected to a switching array including Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) for example, to provide a desired amount of current to a battery to be charged. MOSFETs have some “on” resistance which causes heat to be generated in the MOSFETs due to current flow through semiconductor junctions thereof. This heat can build up, if not properly dissipated, to a point where the MOSFETs can become damaged. Heat however, can be controlled by reducing the amount of current supplied to a battery connected to the charger.
Battery chargers are connected to an AC line circuit through a breaker, such as a 15 Amp breaker, for example and thus it is important not to attempt to draw more current than allowed by the breaker from the AC line circuit. Typically, users of battery chargers have no way of limiting AC line current supplied to a battery charger as most chargers provide few controls and many simply have only a line plug for controlling the operation of the charger. Use of the line plug provides only on/off functions and involves no regard for other circuits that may be supplied by or through the same breaker.
In all battery chargers battery voltage and current must be controlled to avoid damaging the battery being charged. Typically conventional chargers employ circuitry that implements a slow control loop that adjusts the current supplied to the battery to achieve the desired battery voltage. The use of the slow control loop involves producing a current command signal that is shaped to mimic the incoming voltage waveform to produce a high bandwidth AC current command signal to control the current drawn from the AC power source. Since the high bandwidth current command signal mimics the input AC voltage waveform, high power factor is achieved.
However, the above-described methodology only works if the circuit topology permits control of the current. In particular, as long as the instantaneous AC input voltage, divided by the transformer turns ratio, is kept less than the battery voltage, the above methodology can be used to control the current supplied to the battery and maintain a high power factor. Under these conditions, the charger can be operated as a boost converter using either the leakage inductance of the transformer, or a discrete inductor as a boost inductor and the current may be properly controlled.
However, low frequency or hybrid low/high frequency battery chargers (and inverter/chargers) must operate over a wide range of input and output voltage. The turns ratio of the transformer places a limit on the range of input and output voltage over which boost mode (and current control) is possible. When the instantaneous AC input voltage divided by the transformer turns ratio exceeds the battery voltage the battery current is essentially uncontrolled and limited only by parasitic impedances in the AC source, the charger, the battery, and the associated wiring. To avoid this situation, some charger manufacturers employ circuits that adjust the phase angle at which a triac on the AC input is fired, to keep the AC input voltage in an allowable range. However, in this situation only very coarse control of battery current is possible and such control may be unpredictable due to battery and AC source characteristics.